Small quantities of contamination are detrimental to the microchip fabrication process. Contamination in the form of particulates, films or molecules causes short circuits, open circuits, silicon crystal stacking faults, and other defects. These defects can cause the finished microelectronic circuit to fail. Such failures are responsible for significant yield reductions in the microelectronics industry. Yield reductions caused by microcontamination substantially increase processing costs.
Microelectronic circuits require many processing steps. Processing is performed under extremely clean conditions. However, the amount of contamination needed to produce fatal defects in microcircuits is extremely small. For example, an individual particle as small as 100 Angstroms in diameter can result in a killer defect in a modern microcircuit. Microcontamination may occur at any time during the many steps needed to complete the circuit. Therefore, periodic cleaning of the wafers used for microelectronic circuits is needed to maintain economical production yields. Also, tight control of purity and cleanliness in the processing gas is required.
Future microcircuits will have smaller feature sizes and greater complexities, and will require more processing steps. Therefore, in order to maintain economical yields, contamination control techniques in the process gas system and processing environment must be significantly improved and an improved wafer cleaning procedure must be developed.
Several methods are presently used to clean surfaces for the electronics industry. Solvent or chemical cleaning is used to remove contaminant films from surfaces. Since solvents are selective in the materials they can dissolve, an appropriate solvent must be chosen to remove the contamination. Chemical solutions can be combined with Megasonic or Ultrasonic cleaners. These devices impart high energy sonic waves to the surface which can remove organic films, ionic impurities and particles as small as 3000 Angstroms. However, solvent or chemical cleaning requires extremely pure and clean agents. High purity and cleanliness is difficult and/or expensive to achieve in liquid agents. In addition, the agent becomes progressively more contaminated as it is used and must be disposed of periodically. Failure to change the agent periodically causes redeposition of contaminants, which reduces the effectiveness of the cleaning process. Disposal of such agents frequently results in environmental damage. Also, such agents require special safety procedures during handling in order to minimize exposure to operators.
Gas jet cleaning and liquid spray cleaning are presently used to clean relatively large particles from silicon wafers. Gas jets, (e.g., filtered nitrogen jets) are ineffective in removing particles smaller than about 50,000 Angstroms. Smaller particles are more difficult to remove. This is because the adhesive force tending to hold the particle to the surface is proportional to the particle diameter while the aerodynamic drag force by the gas tending to remove the particle is proportional to the diameter squared. Therefore, the ratio of these forces tends to favor adhesion as the particle size shrinks. Also, smaller particles are not exposed to strong drag forces in the jet since they can lie within the surface boundary layer where the gas velocity is low. Liquid jets provide stronger shear forces to remove particles but are expensive and/or difficult to obtain at high purity and may leave contaminating residues after drying. Also, a common liquid spray solvent (Freon TF) is environmentally damaging.
Exposure to ozone combined with ultraviolet light can be used to decompose contaminating hydrocarbons from surfaces. However, this technique has not been shown to remove contaminating particles.
A recently developed cleaning technique involves the use of a carbon dioxide aerosol to "sandblast" contaminated surfaces. Pressurized gaseous carbon dioxide is expanded in a nozzle. The expansion drops the carbon dioxide pressure to atmospheric pressure. The resulting Joule-Thompson cooling forms solid carbon dioxide particles which traverse the surface boundary layer and strike the contaminated surface. In some cases the carbon dioxide forms a soft material which can flow over the surface, displacing particles without leaving a residue. The technique requires extremely clean and pure carbon dioxide. Trace molecular contaminants (eg., hydrocarbons) in the feed gas can condense into solid particulates or droplets upon expansion, causing deposition of new contaminants on the surface. Carbon dioxide is difficult and/or expensive to provide in ultrahigh purity, i.e., with less than parts per million levels of trace impurities. Because of this problem, the carbon dioxide cleaning technique has not yet been shown to be effective in ultraclean (eg., silicon wafer) applications.
The technique of utilizing solid carbon dioxide to remove particulates from a surface is set forth in U.S. Pat. No. 4,806,171.
European Published Application 0 332 356 discloses a cleaning technique using carbon dioxide wherein the purity of the carbon dioxide is enhanced by first vaporizing liquid carbon dioxide, filtering the resulting gas and reliquefying the gas for use as a cleaning agent in the form of dry ice snow.
UK Published Application 2 146 926 A describes a carbon dioxide cleaning media comprising formed solid carbon dioxide, an overlayer of water ice and an entraining jet of compressed air. This technique complicates the possible sources of contamination for a cleaning media which is required to provide high purity cleaning without recontamining the surface being treated with materials carried in the cleaning media.
Equipment for attempting cleaning with carbon dioxide is described in a brochure from Airco Special Gases titled "Spectra-Clean.TM. CO.sub.2 ". The system comprises a submicron filter and conduit attached to a carbon dioxide pressurized gas cylinder with several stages of pressure reduction to provide a directed stream of carbon dioxide snow for cleaning purposes.
An article in Chemical Processing, November 1989, page 54 identifies that a dry ice "Carbon dioxide" system is available for cleaning from Liquid Carbonic identified as a COLD JET.RTM. CLEANING SYSTEM.
In an article contained in Semiconductor International, November 1989, page 16 Mitsubishi's LSI Research and Development Laboratory reports the use of water ice to clean semiconductor wafers. See also Abstract No. 377 titled "Ultraclean Ice Scrubber Cleaning with Jetting Fine Ice Particles" by T. Ohmori, T. Fukumoto, an T. Kato.
An article by Stuart A. Hoenig, "Cleaning Surfaces with Dry Ice" appearing in Compressed Air Magazine, August 1986, pages 22 through 24 describes a device for using carbon dioxide snow in mixture with dry nitrogen gas as a cleaning agent for appropriate surface cleaning.
A dry ice technique is also disclosed by Stuart A. Hoenig, et al. in the article "Control of Particulate Contamination by Thermophoresis, Electrostatics and Dry Ice Techniques" appearing in the Ninth ICCCS Proceedings 1988 Institute of Environmental Sciences, page 671 through 678. The article described various techniques for reduction of contamination in semiconductor and electronic materials. The use of a stream of dry ice particles is also critiqued.
Despite the attempts at providing the thoroughness of cleaning necessary for microelectronic fabrications and materials, the prior art systems predicated upon liquid solvents, carbon dioxide or water-based cleaners suffer from the disadvantage that these substances themselves are considered to be impurities in the microchip fabrication process. For example, present purity specifications for bulk nitrogen shipped to electronics manufacturers permits no more than about 10 parts per billion carbon dioxide and no more than about 50 parts per billion water. When carbon dioxide or water are used as cleaning agents, a significant amount of these substances will remain on the surface as adsorbed contaminants. Many wafer processing steps such as annealing and dopant fusion are performed at high temperatures and are affected by the presence of reactive contaminants. For example, trace amounts of carbon dioxide may decompose during high temperature processing steps and leave deposited carbon on the silicon wafer surface. The carbon will significantly affect the electrical properties of the finished microcircuit.
Carbon dioxide as a cleaning agent is prone to contamination in excess of the requirements of the microelectronic circuit fabricating industry. Carbon dioxide is typically produced by oxidizing natural gas. Considerable levels of impurities remain in the product of this reaction including many unreacted components of the natural gas and byproducts of the reaction. Carbon dioxide may be further purified through adsorption of impurities on molecular sieves, but purity levels better than parts per million are difficult to achieve. Purification through distillation is not practical since typical impurities, such as hydrocarbons, have molecular weights and boiling points near that of carbon dioxide and therefore cannot be separated efficiently. Carbon dioxide can be sold as a gas or liquid, but must be compressed using lubricated pumps. This increases the contamination level of the carbon dioxide. Finally, liquid carbon dioxide is a strong solvent for hydrocarbon lubricants. Therefore it tends to pick these materials up and become more contaminated during transport to the point of use.
The intrinsically higher contamination level of carbon dioxide, especially with regard to hydrocarbons, results in an unacceptable deposit of condensed, oily droplets on the surface of the microelectronic device to be cleaned. The droplets render the carbon dioxide cleaner unacceptable for microelectronic applications. Efforts have attempted to improve the purity level of carbon dioxide feed gas for such cleaning utilities.
It is also known that water ice-based cleaners have been found to cause damage specifically pits to substrates treated during the cleaning process with the particulate water-ice.
The present invention overcomes the drawbacks of the prior art by providing a highly pure and inert particulate aerosol for cleaning substrates and other surfaces to a level of cleanliness required by the microelectronics industry, while avoiding re-contamination by the particles of the cleaning aerosol themselves. This advance in such cleaning as well as other advantages and distinctions will be demonstrated more particularly by the disclosure of the present invention which follows.